A transmitter concept that can be implemented with little complexity for transceivers in mobile radio systems is offered by transmitters which have a modulator that functions according to the known principle of two-point modulation. A PLL (Phase Locked Loop=follow-up synchronization) circuit is used in this case as a frequency synthesizer, and is used for the phase or frequency modulation of a high-frequency signal.
The modulation signal is usually injected into the PLL circuit via a programmable frequency divider contained in the feedback path of the PLL loop. A digital modulation signal is used in this case, and is used for continually reprogramming the digital frequency divider. This form of modulation, which is also referred to as single-point modulation, is known for example from the Patent Specifications U.S. Pat. No. 4,965,531, U.S. Pat. No. 6,008,703 and U.S. Pat. No. 6,044,124.
To achieve low noise of the PLL circuit, the bandwidth of the circuit is made much smaller than is required for the transmission of the modulated data. Therefore, in addition to the purely digital modulation, an analog modulation is also used to compensate for the restricted bandwidth. The simultaneous injection of a digital modulation signal and an analog modulation signal into a PLL circuit is referred to as two-point modulation.
The injection of the digital and analog modulation signals takes place at two different points of the PLL circuit. What is important for the way in which the two-point modulation operates is that, apart from being synchronized in phase, a high degree of coincidence of the amplitudes of the two modulation signals used is required. However, on account of production tolerances of the components for the analog modulation, fluctuations occur in the modulation gradient and the amplitude level of the modulation. For this reason, it is necessary to perform an amplitude compensation between the analog modulation signal and the digital modulation signal once the PLL circuit has been produced.
If temperature influences additionally have to be taken into account, such a compensation has to be carried out before each transmitting operation.
A two-point modulator and a method for phase and frequency modulation with a PLL circuit is described in the German Laid-open Patent Application DE 199 29 167 A1. The modulation in this case takes place in the first instance at a point of the PLL circuit at which high-pass transmission characteristics are obtained for the modulation frequency. In addition, the modulation takes place at a second point of the PLL circuit, at which low-pass transmission characteristics are obtained for the modulation frequency. The modulation with low-pass transmission characteristics takes place digitally in a frequency divider in the feedback path of the PLL circuit.
A know method for compensating a PLL circuit with two-point modulation comprises impressing the two-point modulation of the circuit in the steady state and using an external measuring receiver to receive and demodulate the signal that is sent. Depending on the demodulation result obtained, a compensation of the digital and analog modulation signals is performed. However, on account of the non-linear behavior of the oscillation-generating element—of a voltage-controlled oscillator VCO—of the PLL circuit with regard to the frequency as a function of the control voltage, this compensation must be performed for each channel. For a relatively large number of channels, this results in a correspondingly long measuring time. In addition, the compensation information must be stored in a memory. It can be regarded as a further disadvantage that the influence of temperature changes is not taken into account in this method.
The receiving and demodulating of the signal generated by the PLL circuit can also be performed by the receiving part of the transceiver. However, this increases the complexity of the circuitry in a disadvantageous way, since this would require a complete second PLL circuit in the receiver.
In the document WO 99/07065 there is a description of a compensating method for a PLL that functions according to the principle of two-point modulation. In this method, the PLL is tuned to different frequencies by specifying different divider values N, and the corresponding voltage values at the input of the VCO are recorded. The pairs of values determined in this way are used to calculate the characteristic of the VCO. The characteristic is evaluated by means of a processor, and scaling values for setting the modulation amplitude of the analog modulation signal are thereby determined.